A polyimide film serving as a highly heat-resistant resin was first manufactured in 1960 by DuPont and called KAPTON, which consists of pyromellitic dianhydride (PMDA) and 1,4-diaminodiphenyl ether (DADE).
This is a polymer characterized by a glass transition temperature (Tg) of 420° C. and a thermal decomposition onset temperature (Tm) of 500° C. or more as well as high electric insulation, mechanical strength and chemical resistance so that it is widely used as a material for aerospace vehicles, electric/electronic components, semiconductors, etc. (non-patent document 1: polyimides; D. Wilson, H. D. Steinberger, R. M. Morgenrother; Blackie, New York (1990)).
A polyimide film called “Upilex” manufactured by Ube Industries, Ltd. in 1980 is a heat-resistant film consisting of biphenyltetracarboxylic dianhydride (BPDA) and 1,4-diaminobenzene and having Tg>500° C. and Tm>550° C. (non-patent document 1).
Any heat-resistant polyimide film comparable to KAPTON or Upilex has never been produced since. They are solvent-insoluble polyimides, and any tetracarboxylic dianhydride replacing PMDA or BPDA has not been developed.
KAPTON and Upilex are less soluble in organic solvents, and they are polymerized in anhydrous solvents at low temperatures to synthesize a polyamic acid, which is then cast and heated to form a polyimide film.
Polyamic acids readily decompose in water so that they are poor storage stability. Polyamic acids are difficult to modify because they undergo rapid exchange interaction to form random copolymers when other components are added.
It has been known that a tetracarboxylic dianhydride and an aromatic diamine are polycondensed by heating in an organic polar solvent to directly produce a polyimide (patent document 1: H. Itatani, U.S. Pat. No. 5,202,411 (1993), U.S. Pat. No. 6,627,307 B1 (2003), U.S. Pat. No. 6,890,626 B1 (1995)).
Toluene sulfonic acid was used as a catalyst (patent document 2: A. Berger, U.S. Pat. No. 4,011,297 (1979), U.S. Pat. No. 4,359,572 (1983)).
When a polyimide is synthesized in the presence of an acid catalyst, however, the catalyst must be separated from the polyimide because the presence of the catalyst in the film may cause deterioration.